This invention relates to high speed data communications over wireless links such as radio links. The present invention is further related to determining the nature of the wireless environment which is in use and dynamically selecting appropriate frequency channels for data transmissions which are more likely to accommodate sustained data transmission at high speeds.
Wireless transmissions, and more particularly radio transmissions, are subject to many environmental factors which degrade the effectiveness of the wireless transmissions. One environmental effect is the physical reality that wireless signals reflect off many surfaces, including buildings, mountains, and even moving objects as such vehicles and/or airplanes. When wireless signals bounce off one or more such objects before arriving at a receiving station, the receiving station receives more than one version of the transmitted signal. This is due to the fact that there are often a number of different routes by which the transmitted signal can find its way to the receiving station. For example, the signal may have a direct path and may have another path which bounces off an object such as a mountain, for example. If there a numerous possible paths, then numerous versions of the transmitted signal arrive with differing signal strengths and at different times. Sorting out these numerous signals can be a difficult problem for the receiving apparatus at the receiving station and can cause a type of interference which is often referred to as multipath (multiple path) interference. In the prior art, adaptive equalization has been employed as one means to mitigate against channel degradation effects (for example, multipath interference) caused by environmental factors.
A popular method of adaptive equalization takes a training signal which sets a digital signal processing phase network to sum up the received signal, which contains multiple path signals, as they arrive at the receiver into one coherent signal.
The present invention is directed to a new channel-adaptive wireless method and apparatus that provides for reliable aggregated data transport, particularly for high speed wireless networks. A radio modem is disclosed, which according to the present invention, can be reconfigured in real-time based on wireless (e.g. radio) channel characteristics provided by a built-in broadband propagation path sensor which performs radio channel characterization, on some periodic basis such as prior to each data transmission session and/or at regular intervals during a session. In the preferred embodiment, the propagation environment is measured in the time domain with a pilot signal using spread spectrum techniques, and then transformed and translated into a frequency domain of interest. Frequencies where the propagation is poor are noted and the communication channels are then established at one or more frequency channels where reliable data communications can be maintained. The wireless environment is periodically tested and frequency channel(s) used for data transmission is(are) moved or changed dynamically when appropriate.
Preferably, the pilot signal is phase-shift keyed with a pseudo-random noise (PN) code such that its frequency content covers the entire frequency spectrum of interest. The pilot signal is transmitted at a transmitting site and received at a receiving site where it is correlated, after appropriate demodulation, against an identical PN code in the receiver. The received, demodulated signal is then transformed into the frequency domain to determine which channels have the least impairment in terms of fading in the frequency domain. High priority channels are moved to those frequencies channels which have the least impairment to maintain reliable data communications. Adaptive equalization can still be performed, if desired, but the amount of equalization needed in an individual channel may be substantially reduced.
The present invention has certain advantages over the existing radio modem technologies:
The radio link can be characterized at all frequencies in one measurement. Thus, the entire spectrum can be tested allowing dynamic assignment of channels only in those frequency bands exhibiting low propagation path impairment. Known adaptive equalization techniques may be performed on individual channels. As a result, a very high quality of data communications can be provided which exhibits low bit error rates.
The present invention allows for the pre-selection of propagation channels with lower levels of impairment to relieve excessive adaptive equalization processing. The preferred approach is essentially a frequency division multiplexed (FDM) approach in which the speed requirement of individual channels can be on the order of 100 Mbps for simplified baseband transmitter and receiver circuits, and yet the channels can be multiplexed together to provide a high aggregate data rate for data communications.
The present invention does not require adaptive attune techniques that utilize multiple antenna elements in order to obtain satisfactory results. The adaptive techniques of the present invention involve detecting a signal from multiple received signals and then selecting appropriate signal paths (by selecting appropriate channel frequencies) to reduce multipath impairment. Thus, wireless communication devices can be made compact by using a single set of transmit and receive antennas compared to the prior art practice of using multiple antenna elements for spatial diversity. To attain this compactness, the present invention gives up some spectrum by inhibiting transmission at impaired frequencies, but has the advantage of not requiring multiple antennas.
The present invention offers a means to increase the data transport capability of the radio channel by using more bandwidth-efficient modulation schemes (e.g. M-ary QAM vs. BPSK) for frequency bands with less propagation impairment for data communications.
The radio links with numerous receivers can be characterized in one measurement. For example, the invention can be used with a command post which communicates with units in the field in different areas. The radio modems used at the field units will usually characterize the wireless environment differently. That is, each unit in the field may determine that it can best communicate with the command post on a different sets of acceptable frequency channels. However, since the entire frequency spectrum is tested in one measurement action, all field units learn at the same time the best frequencies for each of them to use.
Once an acceptable set of frequency channels are determined, the party sending data to the other radio modem receiver may use just one acceptable channel for data communication or may use several channels depending on the transmission protocol used and the type of modulation used.
The term xe2x80x9cdata communicationsxe2x80x9d as used herein means any type of data communication, including voice communications (both analog and digital), facsimile, computer data, video, images, and the like.